The present invention relates, in general, to an apparatus for shaping metal sheets.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
British patent specification no. GB 1 490 535 A describes a hot forming process for shaping metal sheets, whereby a metal sheet such as a steel blank is heated and subsequently placed in a pressing tool in which the blank is formed. Hardening takes place while the blank remains still in the pressing tool.
Certain hot-formed steel, in particular of high-strength steel, used for example in B columns (center pillars) of motor vehicles, are normally cut along their edges to maintain the predetermined dimensional tolerance. Moreover, many structures are perforated after the hot forming process. As the hot forming process results in a very hard martensitic configuration, the use of cutting knives to realize the edge cutting and/or perforation of the formed products is wear-intensive and cost-intensive. Other cutting methods, e.g. using laser, are also very cost-intensive.
Thus, it is desirable to provide the concerned areas softer in order allow cuffing the border or making holes. One approach suggests subjecting the hot-formed part to an additional heat treatment to thereby soften up the material structure in those areas that need to be refinished. The need for an additional process step is accompanied however by an increase in costs so that the overall production becomes inefficient. Another approach suggests configuring the hot forming and hardening processes in such a manner that those areas that need refinishing works are cooled down at a slower pace. This approach runs however counter to conventionally designed hot forming tools that are constructed to achieve a quickest possible cool down. An example of this approach is disclosed in U.S. Pat. No. 5,916,389 which describes a press tool provided with inserts or additional heating elements to effect a reduced cooling action in targeted areas during hardening so that the material in these areas becomes softer at the conclusion of the process.
German Offenlegungsschrift DE 101 62 441 A1 describes a method of making motor vehicle parts of sheet metal in a shaping tool, whereby the material flow is controlled by targeted tempering through introducing and/or removing heat. This approach suffers shortcomings because it is very difficult to realize widely different cooling gradients in small transition zones, i.e. within few millimeters, between hard and soft regions in one and the same structural part.
It would therefore be desirable and advantageous to provide an improved apparatus for shaping metal sheets to obviate prior art shortcomings and to enable realization of widely different cooling gradients within narrow boundaries of a formed part.